Following injury to the blood vessel wall and activation of the coagulation cascade, circulating platelets lose their discoid shape, extend psuedopods, and aggregate to initiate the formation of a hemostatic plug. This temporary seal is reinforced with fibrin strans as a gel is formed by the lateral association of long fibrin polymers. Platelet-fibrin interactions, which require metabolic energy, lead to a substantial contraction of the fibrin gel. The objective of this proposal is to arrive at a better understanding of the molecular basis of platelet shape change and aggregation, fibrin(ogen) binding to platelets, and platelet mediated clot retraction. The hypothesis that the binding of fibrinogen (or protofibrils) to platelets forms a molecular bridge which is important in aggregation will be investigated. Sensitive light scattering techniques developed to study fibrin assembly will be adapted to investigate the influence of well defined fibrin polymers on platelet aggregation. The binding of thesemolecules to stimulated platelets will be quanitated with fluorescence polarization and microscopic techniques, as well as with direct binding assays using radiolabelled fibrin(ogen) derivatives. Fibrin assembly will be followed in the presence of platelets and purified platelet proteins to ascertain any interactions which can influence the rate and extent of fiber formation. These studies should lead into the proposed experiments with the Couette elastometer, which can measure with great sensitivity, the contractile force generated by metabolically active platelets. A long-term goal of the proposed research is the development of a reconstituted contractile system, composed of purified platelet protens, which will be undertaken.